1. Field of the Invention
This invention relates to improved abrasive articles comprising a plurality of abrasive grains and a bond system comprising a binder, the binder comprising a blend of an urea-aldehyde resin with a water dilutable resole phenolic resin, the blend being essentially free of organic solvent and catalyzed by an acidic catalyst system.
2. Discussion of Related Art
Abrasive articles typically comprise abrasive grains and may be in the form of a bonded abrasive article (i.e., a grinding wheel), a nonwoven abrasive article, or a coated abrasive article (e.g., sandpaper).
Generally, coated abrasives comprise a backing onto which a plurality of abrasive particles are bonded thereto. In one major form of the coated abrasive, the abrasive particles are secured to the backing by means of a first binder coat, called a make coating, which is adhered to the backing. Abrasive particles are applied while the make coating is in its uncured state, followed by precure of the make coating. A second binder coat, commonly called a size coating, is applied over the make coating and abrasive particles. The purpose of the size coating is to reinforce the abrasive particles.
In another form of a coated abrasive, the abrasive particles are dispersed in a binder to form an abrasive composite, and this abrasive composite is bonded to the backing by means of a binder. Coated abrasives are used in a variety of different applications from gate removal on forged metal parts to finishing eye glasses. Additionally, coated abrasives are converted into a wide variety of different forms including endless belts, sheets, cones, discs, and the like.
Nonwoven abrasives typically comprise a lofty, porous web having abrasive particles adhered thereto by means of a binder.
Bonded abrasives typically comprise a shaped matrix of abrasive particles in a binder.
Binder systems of abrasive articles present challenges to those skilled in the art who want to improve processing and performance of such abrasives. Binder systems employing single resin systems are known. For example, urea-formaldehyde was first patented for use as an adhesive for coated abrasives by Minnesota Mining and Manufacturing Company ("3M") in the mid 1930's (Great Britain Pat. No. 419,812). Since that time, a number of different coated abrasive products have been made with acid catalyzed UF resins. Typical catalysts used with urea-formaldehyde resins are aluminum chloride (AlCl.sub.3) and ammonium chloride (NH.sub.4 Cl).
Although urea-aldehyde resins have enjoyed great success in coated abrasives, the need to reduce the use of solvents and unreacted reactants which contribute to release of volatile organic hydrocarbons (VOC) in the process of making coated abrasives, and the need to increase the quality of the abrasives while maintaining or increasing their level of performance is challenging the industry.
In addition, the appearance to the user of the abrasive article is important. For example, attempts to increase the abrading performance of coated abrasives employing urea-aldehyde resins using aluminum chloride alone as the catalyst, according to known techniques, requires a higher than normal temperature to cure the urea-aldehyde resin, which can lead to edge curling of paper-backed coated abrasives. Excessive curling may lead to an inoperable coated abrasive.
Another type of binder system includes phenolic resins. There are two basic types of phenolic resins: resole and novolak phenolic resins. Curing of resole phenolic resins can be accomplished by alkaline or acid catalysts as disclosed in A. Knop and W. Scheib, Chemistry and Applications of Phenolic Resins, Vol. 3, Springer-Verlog, New York, 1979. Acid catalysts for curing phenolic resins are disclosed, for example, in U.S. Pat. Nos. 4,587,291; 4,904,753; and 5,083,650. The need to reduce emissions of volatile organic compounds, however, is a factor with phenolic resins as well. One approach has been to increase the water compatibility of phenolic resins. J. D. Fisher, in an article entitled "Water Compatible Phenolic Resins" in Proceeding of the American Chemical Society, Division of Polymeric Material: Science and Engineering, No. 65, pp. 275-276 (1991), describes a method of making "water compatible" phenolic resins, their benefits, and their shortcomings.
Although the need to reduce emissions of volatile organic compounds has been recognized for individual resins, there is also a need to reduce such emissions when resins are blended. Such blends are desired to maximize the advantages of the individual resins.
In the manufacture of a coated abrasive article, many factors need to be balanced to make a high performing product. For example, the process conditions have a significant effect on the product performance. Traditionally, when urea-aldehyde resins and phenolic resins have been blended under basic conditions, the resulting blend is highly viscous, which can lead to processing problems, especially with finer grade coated abrasive articles (i.e., coated abrasive articles containing abrasive grains having a particle size of less than 50 micrometers, typically less than 30 micrometers). A high viscosity binder precursor is difficult to coat and can cause "flooding", i.e., excessive filling in between the abrasive grains. In addition, during the manufacture of a coated abrasive article, if the binder precursor is not homogenous, especially when resin blends are used, coating problems can result. Non-homogenous resins result in visual defects and performance defects in the finished product.
One method of making resins compatible is to add an organic solvent, which also lowers the viscosity of the binder precursor. However, organic solvents are not environmentally friendly.
U.S. Pat. No. 4,038,046 (Supkis) discloses an abrasive article having a binder precursor consisting of a blend of an urea-formaldehyde resin with a phenolic resin cured under basic conditions to achieve decreased loading, for example, when grinding certain materials such as wood.
A binder precursor system having processing advantages, such as reduced cure time and temperature, which can reduce or eliminate curl in the finished product of, for example, a coated or nonwoven abrasive article, as well as performance advantages, for example, improved cut and/or improved workpiece appearance is desired.